The present invention relates to a halogen-containing thermoplastic resin composition with improved stability against initial coloration, discoloration on aging and blackening.
The halogen-containing thermoplastic resin, represented by polyvinyl chloride resin, give rise, on heating for melt-molding purposes, to a polyene structure with elimination of hydrochloric acid, undergoing yellowing. To enhance the thermal stability of the resin, it has been common practice to incorporate a metallic soap as a stabilizer in the resin. However, since the incorporation of a stabilizer metallic soap alone may lead to the so-called metal burning in a prolonged melt-molding process, which causes blackening of the resin, it has been generally practiced to add an auxiliary stabilizer, such as polyols (e.g. pentaerythritol), organic phosphorous ester (e.g. triphenylphosphite), epoxy compounds (e.g. bisphenol A type epoxy resin) and so on.
Aside from the above-mentioned technology, Japanese Unexamined Patent Application KOKAI 238345/1985 disclosed a material sharing with the present invention some of resin components, although the prior art is not intended to enhance the thermal stability of halogen-containing thermoplastic resin.
Thus, the above-mentioned patent literature describes a resin composition consisting of (a) a thermoplastic resin (inclusive of polyvinyl chloride resin), (b) an ethylene-vinyl acetate copolymer hydrolysate, and (c) a salt or oxide containing at least one element selected from Groups I, II and III of Periodic Table of the Elements and mentions that this composition assures a marked improvement in compatibility.
In addition, Japanese Unexamined Patent Application KOKAI 69955/1977 discloses a polyvinyl chloride barrier packaging composition consisting essentially of a mixture of polyvinyl chloride and 10 to 30 weight percent, based on polyvinyl chloride, of an ethylenevinyl alcohol copolymer which shares some resin components with the composition of the present invention.
However, the use of a metallic soap as a stabilizer in combination with an auxiliary stabilizer has the disadvantage that where the auxiliary stabilizer is a polyol, the resulting composition is inadequate in compatibility and dispersibility so that the plate-out phenomenon (sticking of the batch to the roll) in the roll mixing stage inevitable, that where the auxiliary stabilizer is an organic phosphorous ester, it is hydrolyzed on absorption of moisture to lose its expected effect in part, and that where an epoxy compound is used as the auxiliary stabilizer, yellowing and plate-out cannot be sufficiently inhibited.
The composition described in Japanese Unexamined Patent Application KOKAI 69955/1985 is claimed to offer improved compatibility but has room for improvement in thermal stability.
The composition described in Japanese Unexamined Patent Application KOKAI 69955/1977 is conductive to reduce oxygen permeability with suppression of water vapor permeation but is poor in melt-moldability and in prevention of discoloration, thus being of no great use in practical applications. Furthermore, since this composition is rich in ethylene-vinyl alcohol copolymer, the inherent characteristics of polyvinyl chloride resin are not available, thus delimiting its range of application.
The present invention has been accomplished to provide a halogen-containing thermoplastic resin composition having markedly improved thermal stability.